1. Field of the Invention
The present invention relates to a method for image formation on an object, a thermal transfer sheet for use in this method, and an intermediate transfer recording medium comprising a receptive layer provided separably on at least one side of a substrate sheet.
2. Background Art
Thermal transfer has hitherto been extensively used as a simple printing method. The thermal transfer is a method which comprises the steps of: putting a thermal transfer sheet, comprising a colorant layer provided on one side of a substrate sheet, on top of a thermal transfer image-receiving sheet optionally provided with an image-receptive layer; and image-wise heating the backside of the thermal transfer sheet by heating means such as a thermal head to selectively transfer the colorant contained in the colorant layer to form an image on the thermal transfer image-receiving sheet.
Thermal transfer methods are classified into thermal ink transfer (hot melt-type thermal transfer) and thermal dye sublimation transfer (sublimation-type thermal transfer). The thermal ink transfer is a method for image formation wherein a thermal transfer sheet comprising a substrate sheet, such as a PET film, bearing thereon a heat-fusion ink layer, formed of a dispersion of a colorant, such as a pigment, in a binder, such as a hot-melt wax or resin, is provided and energy according to image information is applied to heating means such as a thermal head to transfer the colorant together with the binder onto a thermal transfer image-receiving sheet such as paper or plastic sheets. Images produced by the thermal ink transfer have high density and possess high sharpness and are suitable for recording binary images of characters or the like.
On the other hand, the thermal dye sublimation transfer is a method for image formation which comprises the steps of: providing a thermal transfer sheet comprising a substrate sheet, such as a PET film, bearing thereon a dye layer formed of a dye, which is mainly thermally transferred by sublimation, dissolved or dispersed in a resin binder; and applying energy according to image information to heating means such as a thermal head to transfer only the dye onto a thermal transfer image-receiving sheet comprising a substrate sheet, such as paper or a plastic, optionally provided with a dye-receptive layer. The thermal dye sublimation transfer can regulate the amount of the dye transferred according to the quantity of energy applied and thus can form gradation images of which the image density has been regulated dot by dot of the thermal head. Further, since the colorant used is a dye, the formed image is transparent, and the reproduction of intermediate colors produced by superimposing different color dyes on top of each other or one another is excellent. Accordingly, high-quality photograph-like full color images can also be formed with excellent reproduction of intermediate colors by transferring different color dyes, such as yellow, magenta, cyan, and black, onto a thermal transfer image-receiving sheet, so as to superimpose the color dyes on top of each other or one another, from a thermal transfer sheet of the different colors.
Specific applications of the thermal transfer image-receiving sheet used in the thermal transfer method are various, and representative examples thereof include proofs of printing, output of images, output of plans and designs, for example, in CAD/CAM, output of various medical analytical instruments and measuring instruments, such as CT scans and endoscope cameras, alternative to instant photographs, output and printing of photograph-like images of a face or the like onto identification cards or ID cards, credit cards, and other cards, and composite photographs and commemorative photographs, for example, in amusement facilities, such as amusement parks, game centers (amusement arcades), museums, and aquaria.
The diversification of the applications has led to an increasing demand for the formation of a thermally transferred image on a desired object. One method proposed for meeting this demand comprises the steps of: providing an intermediate transfer recording medium comprising a substrate and a receptive layer separably provided on the substrate; providing a thermal transfer sheet comprising a dye layer and a heat-fusion ink layer; transferring the colorants such as dyes or pigments from the thermal transfer sheet onto the receptive layer in the intermediate transfer recording medium to form an image on the receptive layer; and then heating the intermediate transfer recording medium to transfer the receptive layer with the image formed thereon onto an object (Japanese Patent Laid-Open No. 238791/1987).
In the transfer of an image-receptive layer onto an object, in order to surely transfer an image-receptive layer, formed by thermally transferring a colorant such as a sublimation dye or a heat-fusion ink in its part to be transferred, on an object, a method for image formation has been proposed in which a colorant is transferred from a thermal transfer sheet by a thermal transfer recording method onto the image receptive layer face of an intermediate transfer recording medium comprising an image-receptive layer provided separably on one side of a substrate sheet to form an image on the image-receptive layer and the image-receptive layer with the image formed thereon is then transferred onto an object through an adhesive layer (Japanese Patent Laid-Open No. 52522/1995).
In the method for image formation in which the above intermediate transfer recording medium in its receptive layer with an image formed thereon by thermally transferring a sublimation dye or a heat-fusion ink is transferred onto an object, however, the image on the receptive layer is formed by directly transfer of a sublimation dye or a heat-fusion ink. In this case, a release agent is added to the receptive layer from the viewpoint of forming an image without fusion between the receptive layer and the dye layer. When a heat-fusion ink is transferred onto the receptive layer, however, it has been often pointed out that the adhesion and transferability of the heat-fusion ink to the receptive layer are so low that print quality is not good.